Acoustic transducer

ABSTRACT

Transducer for ultrasonic intrusion alarm systems utilizing a flat circular plate as a diaphram operating in a bending mode. The plate is mounted at nodal points to prevent interference with the desired mode of vibration, and a reflective surface is provided behind the plate to reinforce its vibrations. The transducer has a housing removably mounted on a base which can be permanently affixed to a suitable mounting surface.

United States Patent [1 1 Barleen et al.

[ 1 ACOUSTIC TRANSDUCER [75] Inventors: David G. Barleen, Lafayette; RexB.

Peters, Concord, both of Calif.

[73] Assignee: Systron-Donner Corporation,

Concord, Calif.

22 Filed: Feb. 14,1974

211 Appl.No.:442,387

[52] US. Cl. BIO/9.1; 179/110 A; 310/82;

310/81, BIO/8.5 [5 1] Int. Cl H04r 17/00 [58] Field of Search 310/82,8.5, 8.6, 9.1,

310/94; 179/110 A, 181 R, 181 W, 115 R, 138 R; 181/31 R, 32 R; 340/261,384, 388, 391

[56] References Cited UNITED STATES PATENTS 3,578,995 Massa 310/81 XJune 17, 1975 3,736,632 6/1973 Barrow 310/82 X 3,737,690 6/1973 Antonio310/82 3,761,956 9/1973 Takahashi et 310/91 X Primary Examiner-Mark O.Budd Attorney, Agent, or Firm-Flehr, Hohbach, Test, Albritton & Herbert[57] ABSTRACT Transducer for ultrasonic intrusion alarm systemsutilizing a flat circular plate as a diaphram operating in a bendingmode. The plate is mounted at nodal points to prevent interference withthe desired mode of vibration, and a reflective surface is providedbehind the plate to reinforce its vibrations. The transducer has ahousing removably mounted on a base which can be permanently affixed toa suitable mounting surface.

12 Claims, 6 Drawing Figures ACOUSTIC TRANSDUCER BACKGROUND OF THEINVENTION This invention pertains generally to electroacoustictransducers and more particularly to a transducer for use in anultrasonic intrusion alarm system.

In ultrasonic intrusion alarm systems, ultrasonic energy of apredetermined frequency, for example 19.2 KHz is transmitted by one ormore transducers into a room or other area to be protected. The energyis reflected by the walls, floor, ceiling and other objects in theprotected area and received by one or more receiving transducers, If anintruder enters the protected area, the energy reflected by his movingbody undergoes a Doppler frequency shift, and this shift is detected bysuitable equipment connected to the receiving transducer.

The transducers used in ultrasonic alarm systems are commonly mounted onthe ceiling of the room or other area to be protected. One type oftransducer which has been used in such systems in the past is describedin US. Pat. No. 3,287,693, issued Nov. 22, I966 to Samuel M. Bagno andassigned to the assignee herein. Although generally efficient andeffective, the Bagno transducer has a bell shaped diaphram which makesit conspicuous and subject to damage by vandals. These transducersradiate strongly from their edges, producing an intense acoustic fieldparallel to the ceiling or other mounting surface. This edge radiationis of little or no use in detecting intruders, and it can interfere withthe operation of the system by passing directly to a receivingtransducer and producing a strong signal which suppresses the desiredsignals. In addition, the edge radiation can cause undesirable couplingbetween adjacent alarm systems.

Other transducers heretofore provided have a strong axial component ofradiation which is generally reflected directly back to the transducerin a rectangular room and is of little or no use in detecting intruders.

SUMMARY AND OBJECTS OF THE INVENTION The transducer of the inventionutilizes a flat circular plate operated in a bending mode for radiatingor receiving ultrasonic energy. When the transducer is utilized as atransmitter, the energy radiated is concentrated in a conical regionlying midway between the axis and the mounting plane of the transducerwhere it has been found to provide the most effective coverage of theprotected area. The plate is mounted by studs affixed to its nodalpoints, and a reflector mounted behind the plate tends to reinforce thevibrations of the plate. The transducer has a streamlined housing whichmakes it relatively inconspicuous, and the housing mounted on a basefrom which it can be readily re moved to permit access to componentswithin the housing.

It is in general an object of the invention to provide a new andimproved transducer for use in ultrasonic intrusion alarm systems.

Another object of the invention is to provide a transducer of theforegoing character which utilizes a flat circular plate as a diaphramoperating in a bending mode.

Another object of the invention is to provide a transducer of the abovecharacter having a housing removably mounted on a base adapted formounting on a planar surface.

Another object of the invention is to provide a transducer of the abovecharacter in which most of the energy radiated is concentrated in aconical region between the axis and mounting plane of the transducer.

Additional objects and features of the invention will be apparent fromthe following description in which the preferred embodiment is set forthin detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded perspective viewof one embodiment of a transducer according to the invention.

FIG. 2 is a cross-sectional view of the assembled transducer, takenalong line 22 in FIG. 1.

FIG. 3 is an enlarged sectional view of a portion of the transducer ofFIG. 1.

FIG. 4 is a rear elevational view of the circular plate utilized as adiaphram in the transducer of FIG. 1.

FIG. 5 is a circuit diagram of the transducer of FIG. 1 connected foruse as a transmitter.

FIG. 6 is a circuit diagram of the transducer of FIG. 1 connected foruse as a receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENT The transducer includes agenerally circular base 10 which is adapted to be mounted on a planarsurface such as the ceiling of a room. The base can be secured to themounting surface by suitable means such as a two-sided adhesive member11 or mounting screws 12. An opening 13 is provided in the base toaccomodate wires for making electrical connections to the transducer.

A housing I6 is removably mounted on base 10 and releasably securedthereto by resilient latch hooks 17. The latch hooks are attached to thebase, and they engage openings 18 formed in the side wall 19 of thehousing.

Housing 16 includes a front wall 21 having a generally planar outersurface 22 which is highly reflective to the energy to be radiated orreceived by the transducer. An annular lip 23 extends from the outersurface of the front wall and cooperates with this surface to define acavity 24in which a circular plate 26 is mounted. In the embodimentillustrated, front wall 21 and plate 26 are of smaller diameter thanbase 10, and side wall 19 is tapered in the manner shown. The outersurface 27 of plate 26 is generally flush with the outer margin ofanular lip 23, and the distance between the base and the plate issubstantially less than the diameter of the plate whereby the transducerhas a relatively flat, streamlined appearance.

Plate 26 has a diameter greater than several wavelengths of the energyto be radiated or received. In the preferred embodiment, the plate isfabricated of aluminum, and for an operating frequency of 19.2 KHz, ithas a thickness of 0.040 inch and a diameter of 4.56 inches. The bendingwavelength of the 19.2 KHz energy in this material is approximately0.865 inch, and the diameter of the plate is 4.56/.865 or 5.27wavelengths.

A piezoelectric ceramic resonator 31 is bonded to the back side 28 ofplate 26 by a conductive adhesive 32. The resonator is a thin flat diskwhich is polarized through its thickness, i.e. perpendicular to thesurface of the plate. When the transducer is used as a transmitter, theresonator is driven in the d or radial expandor mode to produce acircular bimorph at the center of the plate. The alternately reversingcurvature induced by the resonator at the center of the plate sets up astanding wave condition in the plate. The plate vibrates with standingwave undulations having circular nodes disposed concentrically of theresonator and spaced apart by a distance corresponding to one-half ofthe bending wavelength of the energy radiated or received. A plateoperating at a frequency of 19.2 KHz and having the dimensions givenabove has 5 nodes, designated 1-5 in FIG. 4. The size of the resonatoris not critical, but its thickness is preferably of the same order asthat of the plate and its diameter is preferably slightly less thanone-half wavelength of the bending waves in the plate at the frequencyof operation.

Plate 26 is mounted on the front wall 21 of housing 16 by means of studs36 which are welded to the plate. In the preferred embodiment. threesuch studs are provided, and they are spaced equally along the path ofthe third node from the center of the plate. Rubber grom mets 37 aremounted in openings 38 in the front wall, and the studs extend throughthe grommets. inside the housing, nuts 39 are mounted on the studs toretain them in the grommets. 1n the preferred embodiment, the plate isspaced approximately one-half wavelength from the front wall of thehousing so that the space be tween the plate and wall serves as aresonant chamber which tends to reinforce the vibrations of the plate atthe frequency operation. The same reinforcement can be provided bymaking the spacing any other integral multiple of one-half wavelength.

A component mounting fixture 41 is mounted within housing 16 and securedto wall 21 by flat-headed screws 42. The screws are mounted incountersunk re cesses 43 on the front side of the wall, and they engagecars 44 on the sides of the fixture.

Coupling to resonator 31 is accomplished by components mounted insidefixture 41. These components include a transformer 46, a capacitor 47,and a potentiometer 48. The potentiometer has an adjusting shaft 48awhich is accessable through an opening 49 in side wall 19 of thehousing. A removable plug 51 provides means for closing this opening.

When the transducer is used as a transmitter, potentiometer 48 is notused, and the remaining components are interconnected in the mannerillustrated in FIG. 5. The signal to be radiated is applied to theprimary winding of transformer 46, resonator 31 and capacitor 47 areconnected in parallel across the secondary winding, and one side of thiswinding is grounded. The capacitor and resonator are chosen to havetemperature coefficience of opposite polarities to provide temperaturecompensation and stability. The electrical connections to resonator 31are made by a first lead 56 connected directly to one side of theresonator and a second lead 57 connected to a ground lug 58 mounted onone of the studs 36 affixed to plate 26.

When the transducer is used as a receiver, the components areinterconnected in the manner shown in FIG. 6. As illustrated, resonator31 and capacitor 47 are connected in parallel with the primary windingof transformer 46, and potentiometer 48 is connected across the primaryto provide means for adjusting the sensitivity of the system.

Means is provided to permit housing 16 and the components carriedthereby to be removed from base without disturbing the externalconnections to the transducer whereby the housing and components can beremoved even though the base is installed on a mounting surface andwired permanently in place. For this purpose, a terminal block 61 ismounted on base 10, and external connections are made to this block. Theblock includes connector pins 62 which extend into openings 63 infixture 41 where they mate with connector sockets 64. Permanentconnections between the connector sockets and the components are made ina conventional manner such as by leads 66. Alignment pins 67 carried bybase 10 engage guide openings in fixture ears 44 to assure properalignment and mating of the connector pins and sockets when the housingis on the base.

Operation and use of the transducer can be described brieflyv Thetransducer is installed by mounting base 10 on a suitable surface, suchas a ceiling, with the connecting leads passing through opening 13. Theleads are connected to terminal block 61, and the housing is mounted onthe base where it is secured by latch hooks 17. Thereafter. the housingcan be removed by depressing the latch hooks through openings 18 andwithdrawing the housing from the base.

When the transducer is operated as a transmitter, resonator 31 isenergized at the desired operating frequency, e.g. 19.2 KHZ, producing acircular bimorph at the center of the plate. The alternately reversingcurvature induced by resonator 31 causes plate 26 to vibrate withstanding wave undulations having circular nodes, as illustrated in FIG.4. The nodes are disposed concentrically of the resonator and spacedapart by a distance of one wavelength of the bending waves in the plateat the frequency of operation. The space between plate 26 and housingwall 21 serves as a resonant cavity which tends to reinforce thevibrations of the plate at the frequency of operation.

The radiation pattern produced by the plate is determined by theinterference and reinforcement of the acoustic waves originating fromthe different parts of the plate. All of the patterns are circular andin form of conical rays of sound pressure. The pattern has a low valuealong the transducer axis because the peaks and valleys of the bendingwaves tend to cancel in that direction. Likewise, very little energy isradiated in the plane of the mounting surface because of the combinedeffects of cancellation and relatively poor coupling parallel to theplate. The majority of the acoustic output from the transducer isdirected in a conical region between the axis and the mounting plane.The angle of the cone is determined by the relative wavelengths of theenergy radiated in the plate and in air, and this relationship isdependent on the thickness of the plate. In the preferred embodiment,the plate thickness is such that the wavelength in the plate is on theorder of 1.4 times the wavelength in air, and the conical region isabout midway between the axis and the mounting plane.

When the transducer is used as a receiver, received energy causes theplate to vibrate, producing an output signal from resonator 31. At thefrequency for which the transducer is designed, e.g. 19.2 KHz, theenergy reflected by wall 21 enhances the vibrations, and the output fromthe resonator is maximized. The sensitivity of the transducer can beadjusted by means of potentiometer 48. As in the case of thetransmitter, the sensitivity of the transducer as a receiver is greatestin a conical region between the axis and the mounting plane of thetransducer.

The transducer has a number of important features and advantages. it isrelatively inconspicuous in that it protrudes a relatively shortdistance from the surface on which it is mounted, and it can be mountedin a recessed area to make it even more inconspicuous. The radiationpattern of the transducer makes it ideal for use in an ultrasonicintrusion alarm system. The transducer is readily installed and removed,and the mounting studs do not interfere with the desired vibration ofthe plate since they are attached at nodal points where the motion isessentially zero. Moreover, because of the flat plate, the transducer iseconomical to manufacture.

it is apparent from the foregoing that a new and improved transducer hasbeen provided. While only the presently preferred embodiment has beendisclosed, as will be apparent to those familiar with the art, certainchanges and modifications can be made without departing from the scopeof the invention as defined by the following claims.

We claim:

1. in an acoustic transducer for radiating or receiving energy ofpredetermined frequency: a base, a housing having a wall with agenerally planar outer surface removably mounted on the base, matingelectrical connectors carried by the base and housing for makingelectrical connections to components mounted in the housing, asubstantially flat circular plate mounted outside the housing and spacedfrom the outer surface of the wall by a distance on the order of anintegral number of half wavelengths of the energy to be radiated orreceived, a piezoelectric ceramic resonator affixed centrally to oneside of the plate, and mounting means extending between the plate andthe wall, said mounting means engaging the plate at a plurality ofpoints spaced from the center of the plate by a distance correspondingto an integral multiple of one-half wavelength of energy of thepredetermined frequency in the plate.

2. The transducer of claim I further including an anular lip extendingfrom the outer surface of the wall and cooperating with said surface todefine a cavity in which the plate is mounted.

3. The transducer of claim 1 the housing is removably mounted on thebase and mating electrical connectors are carried by the base andhousing for making electrical connections to components mounted in thehousmg.

4. The transducer of claim I further including latch members forreleasably securing the housing to the base.

5. The transducer of claim 4 wherein the latch members comprise latchhooks carried by the base for releasably engaging openings in a wall ofthe housing.

6. The transducer of claim 1 wherein the base comprises a generallyplanar circular member disposed on the side of the housing opposite theplate and adapted to be mounted on a generally planar surface such asthe ceiling of a room, the plate being spaced from the base member by adistance substantially less than the diameter of the plate,

'7. The transducer of claim 6 wherein the diameter of the base member isgreater than the diameter of the plate and the housing includes atapered side wall extending between the base member and the wall withthe planar surface.

8. In an acoustic transducer for radiating or receiving energy ofpredetermined frequency: a generally flat circular plate having adiameter greater than several wavelengths of the energy to be radiatedor received, a piezoelectric ceramic resonator having a flat faceaffixed centrally to one side of the plate, the plate being vibratedwith standing wave undulations having circular nodes disposedconcentrically of the resonator and spaced apart by a distancecorresponding to one-half wavelength of the energy radiated or received,a plurality of mounting studs attached to the plate along one of thenodes, resilient grommets mounted on the reflective member, saidgrommets having openings through which the studs extend, and meansengaging the studs on the side of the grommets opposite the plate forretaining the studs in the grommets.

9. The transducer of claim 8 further including a generally planarreflective member on one side of the plate and spaced therefrom by adistance corresponding to an integral number of half wavelengths of theenergy radiated or received.

10. The transducer of claim 8 wherein the studs are attached to theplate at the third node from the center of the plate.

11. in an acoustic transducer for radiating or receiving energy ofpredetermined frequency: a base, a housing having a wall with agenerally planar outer surface mounted on the base, a substantially flatcircular plate mounted outside the housing and spaced from the outersurface of the wall by a distance on the order of an integral number ofhalf wavelengths of the energy to be radiated or received, apiezoelectric ceramic resonator affixed centrally to one side of theplate, a plurality of studs affixed to the plate at points spaced fromthe center of the plate by a distance corresponding to an integralmultiple of one-half wavelengths of energy of the predeterminedfrequency in the plate, resilient grommets mounted in openings of thehousing wall, said grommets having openings through which the studsextend, and means engaging the studs inside the housing for retainingthe studs in the grommets.

ii. in an acoustic transducer for radiating or receiving energy ofpredetermined frequency: a base. a housing having a wall with agenerally planar outer surface mounted on the base, a substantially flatcircular plate mounted outside the housing and spaced from the outersurface of the wall by a distance on the order of an integral number ofhalf wavelengths of the energy to be radiated or received, said plateoperating in a bending mode wherein the wavelength of bending waves inthe plate at the predetermined frequency is greater than the wavelengthof radiation in the air corresponding to the bending waves. apiezoelectric ceramic reso= nator affixed centrally to one side of theplate. and mounting means extending between the plate and the housing ata plurality of points spaced from the center of the plate by a distancecorresponding to an integral multiple of one=half wavelength of thebending waves at the predetermined frequency.

i i i i i

1. In an acoustic transducer for radiating or receiving energy ofpredetermined frequency: a base, a housing having a wall with agenerally planar outer surface removably mounted on the base, matingelectrical connectors carried by the base and housing for makingelectrical connections to components mounted in the housing, asubstantially flat circular plate mounted outside the hoUsing and spacedfrom the outer surface of the wall by a distance on the order of anintegral number of half wavelengths of the energy to be radiated orreceived, a piezoelectric ceramic resonator affixed centrally to oneside of the plate, and mounting means extending between the plate andthe wall, said mounting means engaging the plate at a plurality ofpoints spaced from the center of the plate by a distance correspondingto an integral multiple of one-half wavelength of energy of thepredetermined frequency in the plate.
 2. The transducer of claim 1further including an anular lip extending from the outer surface of thewall and cooperating with said surface to define a cavity in which theplate is mounted.
 3. The transducer of claim 1 the housing is removablymounted on the base and mating electrical connectors are carried by thebase and housing for making electrical connections to components mountedin the housing.
 4. The transducer of claim 1 further including latchmembers for releasably securing the housing to the base.
 5. Thetransducer of claim 4 wherein the latch members comprise latch hookscarried by the base for releasably engaging openings in a wall of thehousing.
 6. The transducer of claim 1 wherein the base comprises agenerally planar circular member disposed on the side of the housingopposite the plate and adapted to be mounted on a generally planarsurface such as the ceiling of a room, the plate being spaced from thebase member by a distance substantially less than the diameter of theplate.
 7. The transducer of claim 6 wherein the diameter of the basemember is greater than the diameter of the plate and the housingincludes a tapered side wall extending between the base member and thewall with the planar surface.
 8. In an acoustic transducer for radiatingor receiving energy of predetermined frequency: a generally flatcircular plate having a diameter greater than several wavelengths of theenergy to be radiated or received, a piezoelectric ceramic resonatorhaving a flat face affixed centrally to one side of the plate, the platebeing vibrated with standing wave undulations having circular nodesdisposed concentrically of the resonator and spaced apart by a distancecorresponding to one-half wavelength of the energy radiated or received,a plurality of mounting studs attached to the plate along one of thenodes, resilient grommets mounted on the reflective member, saidgrommets having openings through which the studs extend, and meansengaging the studs on the side of the grommets opposite the plate forretaining the studs in the grommets.
 9. The transducer of claim 8further including a generally planar reflective member on one side ofthe plate and spaced therefrom by a distance corresponding to anintegral number of half wavelengths of the energy radiated or received.10. The transducer of claim 8 wherein the studs are attached to theplate at the third node from the center of the plate.
 11. In an acoustictransducer for radiating or receiving energy of predetermined frequency:a base, a housing having a wall with a generally planar outer surfacemounted on the base, a substantially flat circular plate mounted outsidethe housing and spaced from the outer surface of the wall by a distanceon the order of an integral number of half wavelengths of the energy tobe radiated or received, a piezoelectric ceramic resonator affixedcentrally to one side of the plate, a plurality of studs affixed to theplate at points spaced from the center of the plate by a distancecorresponding to an integral multiple of one-half wavelengths of energyof the predetermined frequency in the plate, resilient grommets mountedin openings of the housing wall, said grommets having openings throughwhich the studs extend, and means engaging the studs inside the housingfor retaining the studs in the grommets.
 12. In an acoustic transducerfor radiating or receiving energy of predetermined frequency: a base, ahousing having a wall with a generally planar outer surface mounted onthe base, a substantially flat circular plate mounted outside thehousing and spaced from the outer surface of the wall by a distance onthe order of an integral number of half wavelengths of the energy to beradiated or received, said plate operating in a bending mode wherein thewavelength of bending waves in the plate at the predetermined frequencyis greater than the wavelength of radiation in the air corresponding tothe bending waves, a piezoelectric ceramic resonator affixed centrallyto one side of the plate, and mounting means extending between the plateand the housing at a plurality of points spaced from the center of theplate by a distance corresponding to an integral multiple of one-halfwavelength of the bending waves at the predetermined frequency.